Burners

ABSTRACT

The invention provides a nozzle-mixed burner for use in an endothermic process such as hydrogen reforming or ammonia reforming or ethylene cracking or EDC cracking. The burner comprises a fuel duct extending axially through a wall of the furnace. The fuel duct delivers gaseous fuel to an array of nozzles extending laterally to spray the fuel outwardly into an annular chamber defined by a cap. Combustion air is delivered to the chamber by way of a primary air duct. A convergent-divergent section of the duct and an outwardly divergent section of the chamber together form a radially extending venturi. Combustion of the fuel, supplemented by the venturi effect, draws the air through the duct by natural inspiration.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority from United Kingdom Patent Application No. 10 14 969.8, filed Sep. 9, 2010 the entire disclosure of which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to burners, particularly but not necessarily burners for use in radiant-wall side-fired furnaces of the kind used in endothermic processes such as hydrogen reforming, ammonia reforming, ethylene cracking and ethylene dichloride (EDC) cracking.

2. Description of the Related Art

Considering hydrogen reforming as an example, this is an industrial process for the production of hydrogen by reacting hydrocarbons such as natural gas with steam in the presence of a catalyst. In the primary reaction, methane reacts with water to yield carbon monoxide plus hydrogen:

CH₄+H₂O→CO+3H₂  [Equation 1]

The reaction is highly endothermic, and therefore the process is carried out in a furnace, typically with wall-mounted burners, to maintain a temperature of typically 1050° C. to 1200° C. The burners are commonly fuelled with natural gas, liquefied petroleum gas (LPG) or refinery gas which may contain varying amounts of hydrogen, and sometimes additionally with residual hydrocarbon gas from the reforming process, sometimes blended with or supported by other fuels such as methane. At start-up, natural gas (which is mostly methane) or LPG is usually used as fuel.

NOx formation may be controlled by staging technology in which the supplies of fuel and/or combustion air are adjusted. It follows that the burners are required to handle a range of fuels, and over recent years this range has been extended by a trend towards the use of fuels containing a higher proportion of hydrogen.

The operation of an industrial reformer or cracker gives rise to two notable and related problems, flashback and noise, as will now be discussed.

Hitherto, hydrogen reforming (and other endothermic processes such as ammonia reforming, ethylene cracking and EDC cracking) has used pre-mix burners in which a jet of fuel gas is injected at the outer end of a venturi tube extending through the wall of the furnace and in the venturi tube naturally inspired combustion air is mixed with the fuel to form a combustible mixture ignited at the inner end of the tube, in the furnace. Fuels with relatively high flame speeds, such as those containing relatively high proportions of hydrogen which are now increasingly used, may cause a flashback. At a minimum, this reduces the performance of the plant; and if it results in damage to the burner, the cost of repair or replacement is considerable, especially if the plant has to be shut down. With hundreds of burners in a typical furnace, the risk of flashback in at least one is high.

Hundreds of burners create a great deal of noise, from the combustion itself and from the delivery of combustion air, and the fact that the venturi tubes of the conventional burners provide hundreds of essentially straight passages through the wall of the furnace exacerbates the impact of this noise on the environment of the plant.

Generally, permitted noise levels are limited by environmental controls and/or health and safety regulations. To keep reformer noise levels within such limits it has been necessary to fit a silencer in the form of a sound enclosure on the outer end of each burner. However, in conventional burners this constrains the flow of combustion air, so the risk of flashback is increased.

The constriction in the flow of combustion air can be countered by providing a fan to deliver a forced draught, rather than natural inspiration. However this increases both capital cost and running cost, and also introduces another source of noise outside the furnace.

BRIEF SUMMARY OF THE INVENTION

It is a primary object of the present invention to enable a furnace to operate with a range of fuel gases wider than hitherto, including particularly gases with a higher concentration of hydrogen than hitherto. At the same time the invention facilitates operation without forced draught while meeting permitted noise levels and substantially avoiding a risk of flashback.

Thus according to a first aspect of the invention there is provided a burner for an endothermic process (e.g. hydrogen reforming or ammonia reforming or ethylene cracking or EDC cracking), which burner comprises a fuel duct having an axially extending portion extending axially forwards from a proximal end to discharge fuel at a distal end and an air duct having an axially extending portion extending axially forwards from said proximal end to discharge combustion air at said distal end, wherein after discharge the discharged fuel burns in the presence of the discharged combustion air and draws the combustion air through the air duct by natural inspiration, wherein the fuel duct has at said distal end a laterally extending portion whereby the fuel is discharged laterally and the air duct has at said distal end a laterally extending portion whereby the combustion air is discharged laterally.

Preferably the air duct has an axially extending portion circumjacent an axially extending portion of the fuel duct.

Preferably the air duct comprises at its distal end an annular chamber extending laterally outwards towards a lateral opening for the discharge of the combustion air. The annular chamber may comprise a dished section facing rearwards to turn the combustion air from an axial flow to a lateral flow, and it may be divergent towards its lateral opening. Also, the air duct may adjacent its distal end comprise a section convergent towards the annular chamber.

Preferably the fuel duct comprises at its distal end a plurality of ports extending within the annular chamber towards the lateral opening thereof. These ports may comprise a first set of ports for a main fuel supply and a second set of ports for a stabilising fuel supply, and the ports of the second set may be inclined rearwardly of the ports of the first set. The fuel duct may comprise one fuel passage connected to the first set of ports and a separate second fuel passage connected to the second set of ports and there may also be a third set of ports for fuel staging.

In another aspect, and in contrast with the general use hitherto of pre-mix burners, the invention provides a furnace for an endothermic process, wherein the furnace comprises a plurality of nozzle-mix burners configured and arranged to draw in combustion air by natural inspiration. The nozzle-mix burners are preferably burners according to the first aspect of the invention with the air and fuel ducts thereof extending through a wall of the furnace. The furnace may be configured and arranged to operate within a specified noise limit, i.e. so that meets permitted noise levels without a substantial risk of flashback.

In a third aspect the invention provides a method of operating a furnace for an endothermic process, wherein the furnace is heated by a plurality of nozzle-mix burners configured and arranged to draw in combustion air by natural inspiration, and the nozzle-mix burners may be burner according to the first aspect of the invention with the air and fuel ducts thereof extending through a wall of the furnace.

In a fourth aspect the invention provides a method of modifying a furnace equipped with a plurality of wall-mounted burners of the premix type, wherein said method comprises replacing the pre-mix burners with nozzle-mix burners according to the first aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only with reference to the accompanying drawings, which are purely schematic and not to scale, and in which:

FIG. 1 shows in longitudinal cross-section a previously known burner for hydrogen reforming and like endothermic processes;

FIG. 2 shows in longitudinal cross-section a burner for an endothermic process according to the present invention;

FIG. 3 is an end elevation as viewed at X-X in FIG. 2; and

FIG. 4 is a longitudinal cross-section of a nozzle arrangement for supply of two gas fuels.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

Referring first to FIG. 1, this shows a previously known burner indicated generally at 10 mounted in a wall 12 of a furnace for an endothermic process such as hydrogen reforming or ammonia reforming or ethylene cracking or EDC cracking. For simplicity and clarity of illustration, no attempt is made in the drawings to detail the construction of the furnace, including refractory brick or tile material carrying the burner 10 or practical operating features such as valves and igniters, all of which will be comprehended by those skilled in the science. It will also be understood that there will be hundreds of burners like the burner 10 in a typical furnace for endothermic processes.

The burner 10 is of the pre-mix type. It comprises a venturi tube 14 extending through the wall 12 from an outer, proximal, end to an inner, distal, end. At its proximal end gaseous fuel G (indicated in the drawings by dark arrows) is injected into the venturi tube 14 by way of a jet 16. Combustion air A (indicated by white arrows) is naturally inspirated into the venturi tube 14 wherein it mixes with the fuel G to form a combustible mixture M (indicated by light arrows). The fuel-air mixture M flows through the venturi tube 14 to burn at its proximal end as indicated schematically at F.

Burners such as the burner 10 are widely used in furnaces for endothermic processes such as hydrogen reforming, and have proved generally satisfactory. However we have identified two problems with them. First, they must work with a range of fuels—typically natural gas or LPG at start-up and subsequently refinery gas of varying composition or some byproduct of the reforming process such as, in the case of hydrogen reforming, tail gas residual to pressure swing adsorption (PSA) purification of the hydrogen—and fuels with a relatively high flame speed increase the risk of flashback. Gases containing hydrogen generally have a higher flame speed than natural gas and so give rise to more flashback incidents when used as a fuel—reducing plant performance, at a minimum, and adding costs in rectification. The second problem with burners such as the burner 10 is that they emit a lot of noise. With a view to containing this, the burner 10 has a silencer 18 fitted around the proximal end of the venturi tube 14. The silencer 18 is effective in containing the noise of operation, but it adds to the cost of each burner and the inevitable constriction of the air flow A further increases the risk of flashback.

FIG. 2 illustrates a burner according to the invention designed to tackle these problems of the prior art. Referring to FIG. 2, the burner is indicated generally at 20, mounted in a wall 22 of a furnace. The burner 20 comprises a fuel duct 24 extending generally axially through the wall 22 (carried by a refractory brick or tile not detailed) from an outer, proximal, end to an inner, distal, end. Gaseous fuel G is fed through the fuel duct 24 to burn at the distal end. Thus the burner 20 is of the nozzle-mix kind, and this is a significant difference from the prior art pre-mix burner illustrated by FIG. 1.

At the distal end of the fuel duct 24 is an array of nozzles indicated at 26 each extending radially outwards, i.e. laterally of the axis Y-Y of the fuel duct 24. The nozzles, which are arranged to spray fuel G outwards as indicated at 28, comprise two sets: a first set 26 a of twelve main gas nozzles equiangularly disposed around the fuel duct 24, as shown in FIG. 3, and substantially normal to the axis Y-Y (FIG. 2); and a second set 26 b of twelve stabilising gas nozzles equiangularly disposed around the fuel duct 24 (FIG. 3) and inclined rearwardly (FIG. 2). (It is to be understood that there may be more than twelve nozzles in a set, or fewer; the sets may comprise different numbers; and it may not be necessary to incline one set. However we have found that the configuration shown works effectively). Also as shown in FIG. 3, the stabilising set 26 b is angularly offset with respect to the main set 26 a, so that viewed end-on as in FIG. 3 the nozzles are intercalated.

Circumjacent the fuel duct 24 and coaxial with it is a primary air duct 30 at the distal end of which is a convergent-divergent section 32. The primary air duct 30 delivers combustion air A to the distal end of the burner 20 so that the spray 28 of fuel G can be ignited (by means not detailed) and burn as indicated at F.

A cap 36 is mounted on the distal end of the fuel duct 24. The cap 36 extends across the distal end of the primary air duct 30 and spaced apart from it to form therewith a generally annular chamber 40, laterally open around its outer periphery, into which the fuel G is sprayed by the nozzles 26. The cap 36 is dished at 42 to help turn the air A into and through the annular chamber 40.

As indicated at 44, the annular chamber 40 diverges outwardly so that it forms, with the convergent-divergent section 32 of the primary air duct 30, a radially extending venturi. This venturi supplements the entrainment effect of the spray 28 and the combustion of the fuel G to draw the air A through the primary air duct 30 by natural inspiration.

Around the periphery of the annular chamber 40 and on each side thereof is a recess 46 bounded by a lip or ring 48 that serves to stabilise combustion when fuels of relatively low flame speed are used, e.g. at furnace start-up.

Circumjacent the primary air duct 30 and coaxial with it is a secondary air duct 50 for delivering secondary air S to the combustion zone. The primary air duct 30 and the secondary air duct 50 each contain air control vanes 34.

Those skilled in the art will appreciate from the foregoing description that a burner 20 according to the present invention operates by nozzle-mixing rather than premixing as in prior art burners like those of FIG. 1. If there is any flashback, therefore, this cannot extend upstream of the nozzles 26, so any consequential damage will be limited. Nozzle-mixing is not new per se, but it has hitherto generally required a forced draught supply of combustion air, with extra cost and complexity (especially so where there are hundreds of burners, as in a furnace for hydrogen reforming). But the present invention is configured and arranged—in particular by way of its radial venturi, so that it does not require forced draught. The radially extending nozzles 26 create a plurality of high velocity gas jets that induce combustion air A into the primary air duct 30, and this process is enhanced by the effect of the radial venturi, which creates a reduction of pressure at the entry point for the air. Secondary air S is drawn into the furnace, by way of the secondary air duct 50, by negative operating pressure in the furnace, supplemented by the radially outward thrust of the primary combustion F, which further reduces pressure at the entry point for the air.

By the above means the present invention secures the benefits of nozzle-mixing in limiting flashback without the need for forced draught air supply. However those skilled in the science will appreciate that the invention is not limited to the specific details set forth above, and in general terms it provides a nozzle-mixed burner with air supplied by natural inspiration.

FIG. 4 illustrates a nozzle arrangement for the present invention configured and arranged to receive two gas streams. Main gas nozzles 60 extend radially outwards from a first plenum 62 at the distal end of an inner fuel duct 64 which is supplied with a first gas stream G1 such as tail gas from hydrogen reforming. Stabilising gas nozzles 66 extend radially outwards, and somewhat rearwardly, from a second plenum 68 at the distal end of an outer fuel duct 70 which is supplied with a second gas stream G2 to support combustion of the first gas steam G1.

FIG. 4 also shows staging ports 72 extending outwardly from the first plenum 62 to deliver main fuel G1 therefrom to create a fuel-rich zone for fuel staging. The fuel duct 24 of FIG. 2 may be provided with similar staging ports at its distal end, beyond the cap 36.

Other possible modifications will be apparent to those skilled in the science. 

1. A burner for an endothermic process, which burner comprises a fuel duct having an axially extending portion extending axially forwards from a proximal end to discharge fuel at a distal end and an air duct having an axially extending portion extending axially forwards from said proximal end to discharge combustion air at said distal end, wherein after discharge the discharged fuel burns in the presence of the discharged combustion air and draws the combustion air through the air duct by natural inspiration, wherein the fuel duct has at said distal end a laterally extending portion whereby the fuel is discharged laterally and the air duct has at said distal end a laterally extending portion whereby the combustion air is discharged laterally.
 2. A burner as claimed in claim 1, wherein the axially extending portion of the air duct is circumjacent the axially extending portion of the fuel duct.
 3. A burner as claimed in claim 1, wherein the air duct comprises at its distal end an annular chamber extending laterally outwards towards a lateral opening for the discharge of the combustion air.
 4. A burner as claimed in claim 3, wherein the annular chamber comprises a dished section facing rearwards to turn the combustion air from axial flow through the axially extending portion of the air duct to lateral flow through the laterally extending portion of the air duct.
 5. A burner as claimed in claim 4, wherein the annular chamber is divergent towards its lateral opening.
 6. A burner as claimed in claim 5, wherein the axially extending portion of the air duct comprises adjacent said distal end a section convergent towards the annular chamber.
 7. A burner as claimed in claim 3, wherein the fuel duct comprises at its distal end a plurality of ports extending within the annular chamber towards the lateral opening thereof.
 8. A burner as claimed in claim 7, wherein said plurality of ports comprises a first set of ports for a main fuel supply and a second set of ports for a stabilising fuel supply.
 9. A burner as claimed in claim 8, wherein the ports of said second set are inclined rearwardly of the ports of the first set.
 10. A burner as claimed in claim 8, wherein the fuel duct comprises one fuel passage connected to the first set of ports and a separate second fuel passage connected to the second set of ports.
 11. A burner as claimed in claim 8, wherein said plurality of ports comprises a third set of ports for fuel staging.
 12. A burner as claimed in claim 3, wherein the lateral opening of the annular chamber comprises a flame stabilisation ring.
 13. A burner as claimed in claim 1, wherein the burner comprises a plurality of air control vanes extending across a convergent-divergent section of the axially extending portion of the air duct.
 14. A burner as claimed in claim 1, wherein the burner comprises a secondary air duct extending axially from said proximal end to said distal end.
 15. A furnace for an endothermic process, wherein the furnace comprises a plurality of nozzle-mix burners configured and arranged to draw in combustion air by natural inspiration.
 16. A furnace as claimed in claim 15, wherein said burners are as claimed in claim 1 each with the air duct and the fuel duct thereof extending through a wall of the furnace.
 17. A furnace as claimed in claim 16, wherein the furnace is configured and arranged to operate within a specified noise limit.
 18. A method of operating a furnace for an endothermic process, wherein the furnace is heated by a plurality of nozzle-mix burners configured and arranged to draw in combustion air by natural inspiration.
 19. A method of operating a furnace as claimed in claim 18, wherein said burners are as claimed in claim 1 each with the air duct and the fuel duct thereof extending through a wall of the furnace.
 20. A method of modifying a furnace equipped with a plurality of wall-mounted burners of the premix type, wherein said method comprises replacing the pre-mix burners with nozzle-mix burners as claimed in claim
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